Microspheres including nanoparticles

ABSTRACT

A microparticle can include a central region and a peripheral region. The peripheral region can include a nanoparticle, such as a metal nanoparticle, a metal oxide nanoparticle, or a semiconductor nanocrystal. The microparticle can be a member of a monodisperse population of particles.

FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The U.S. Government may have certain rights in this invention pursuantto Grant No. DMR-98-1328 awarded by the National Science Foundation andGrant No. NSF-CRC CHE-0209898.

TECHNICAL FIELD

This invention relates to microspheres.

BACKGROUND

Microspheres containing chromophores are used in a variety ofapplications. In many of these applications, the photostability of thechromophores and the monodispersity of the microspheres are important.

SUMMARY

In general, a microsphere includes a central region and a peripheralregion that includes a nanoparticle. The nanoparticle can include ametal nanoparticle, a metal oxide nanoparticle, or a semiconductornanocrystal.

In one aspect, a microsphere includes a central region and a firstperipheral region on a surface of the central region, the firstperipheral region including a first nanoparticle.

In another aspect, a microsphere includes a central region including afirst inorganic material and a peripheral region on a surface of thecentral region, the peripheral region including a second inorganicmaterial and a semiconductor nanocrystal covalently linked to the secondinorganic material.

In another aspect, a population of microspheres includes a membermicrosphere including a central region including a first inorganicmaterial, and a first peripheral region on a surface of the centralregion, the first peripheral region including a second inorganicmaterial and a first nanoparticle.

The central region can include an inorganic material. The central regioncan include silicon. The first peripheral region can include aninorganic material. The first peripheral region can include silicon ortitanium. The microsphere can have a diameter of less than 500micrometers, less than 10 micrometers, or less than 1 micrometer. Thecentral region can be substantially free of nanoparticles. The firstnanoparticle can be a metal nanoparticle, a metal oxide nanoparticle, ora semiconductor nanocrystal, such as a gold nanoparticle, a cobaltnanocrystal, an iron oxide nanocrystal, or a Group II-VI nanocrystal.

The semiconductor nanocrystal can include a core including a firstsemiconductor material. The semiconductor nanocrystal can furtherinclude a shell overcoating the core, the shell including a secondsemiconductor material. The first semiconductor material can be a GroupII-VI compound, a Group II-V compound, a Group III-VI compound, a GroupIII-V compound, a Group IV-VI compound, a Group I-III-VI compound, aGroup II-IV-VI compound, or a Group II-IV-V compound. The firstsemiconductor material can be ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, HgS,HgSe, HgTe, AlN, AlP, AlAs, AlSb, GaN, GaP, GaAs, GaSb, GaSe, InN, InP,InAs, InSb, TlN, TlP, TlAs, TlSb, PbS, PbSe, PbTe, or mixtures thereof.The second semiconductor material can be ZnO, ZnS, ZnSe, ZnTe, CdO, CdS,CdSe, CdTe, MgO, MgS, MgSe, MgTe, HgO, HgS, HgSe, HgTe, AlN, AlP, AlAs,AlSb, GaN, GaP, GaAs, GaSb, InN, InP, InAs, InSb, TlN, TlP, TlAs, TlSb,TlSb, PbS, PbSe, PbTe, or mixtures thereof.

The microsphere can include a plurality of semiconductor nanocrystalshaving a rms deviation in diameter of no greater than 5%. Themicrosphere can include a ligand having affinity for a surface of asemiconductor nanocrystal. The ligand can be covalently linked to thefirst peripheral region. The central region can be substantiallyspherical in shape. The microsphere can be a member of a population ofmicrosphere having a rms deviation in diameter of no greater than 10%.

The microsphere can include a second peripheral region on a surface ofthe first peripheral region. The second peripheral region can include aninorganic material. The second peripheral region can include silicon ortitanium. The second peripheral region can include a secondnanoparticle. The second nanoparticle can be a metal nanoparticle, ametal oxide nanoparticle, or a semiconductor nanocrystal. In certainembodiments, when the first nanoparticle is a fluorescent nanoparticle,the second nanoparticle can be a fluorescent nanoparticle. The firstnanoparticle can have an emission distinguishable from an emission ofthe second nanoparticle.

In another aspect, a family of microsphere populations includes a firstpopulation of microspheres, where each microsphere in the firstpopulation includes a central region, and a peripheral region on asurface of the central region, the peripheral region including a firstnanoparticle; and a second population of microspheres, where eachmicrosphere in the second population includes a central region; and aperipheral region on a surface of the central region, the peripheralregion including a second nanoparticle.

In the family, the first nanoparticle and the second nanoparticle caneach independently be a fluorescent nanoparticle. When the first and thesecond nanoparticles are both fluorescent nanoparticles, the firstpopulation can have a fluorescence emission distinguishable from afluorescence emission of the second population.

The microspheres of the first population can have a rms deviation indiameter of no greater than 10%. The microspheres of the secondpopulation can have a rms deviation in diameter of no greater than 10%.The microspheres of the first population can have an average diameterdistinct from an average diameter of the microspheres of the secondpopulation. The microspheres of the first population and themicrospheres of the second population can have distinct fluorescenceemission wavelengths.

The microspheres of the first population can have an average diameter ofless than 500 micrometers, less than 10 micrometers, or less than 1micrometer. The microspheres of the second population can have anaverage diameter of less than 1 micrometer.

In another aspect, a method of making a microsphere includes providing aparticle including a first inorganic material, contacting the particlewith a precursor to a second inorganic material and a firstnanoparticle, and forming a peripheral region on a surface of theparticle, the peripheral region including the second inorganic materialand the first nanoparticle.

The first inorganic material can include silicon. The second inorganicmaterial can include silicon or titanium. The first nanoparticle can bea metal nanoparticle, a metal oxide nanocrystal, or a semiconductornanocrystal. The semiconductor nanocrystal can include a ligand havingaffinity for a surface of a semiconductor nanocrystal. The ligand can becapable of reacting with the precursor to the second inorganic material.

The particle including the first inorganic material can be a member of apopulation of particles having a rms deviation in diameter of no greaterthan 10%. The particle including the first inorganic material can besubstantially spherical in shape. The particle including the firstinorganic material can have a diameter of less than 500 micrometers,less than 10 micrometers, or less than 1 micrometer. The method caninclude forming the particle including the first inorganic material.

In another aspect, a method of making a family of microspherepopulations includes selecting a first nanoparticle and a secondnanoparticle, and forming a first population of microspheres. The firstpopulation includes a first member microsphere including a centralregion including a first inorganic material, and a peripheral region ona surface of the central region, the peripheral region including asecond inorganic material and the first nanoparticle. The methodincludes forming a second population of microspheres. The secondpopulation includes a second member microsphere including a centralregion including a third inorganic material, and a peripheral region ona surface of the central region, the peripheral region including afourth inorganic material and the second nanoparticle.

In the method, the first nanoparticle and the second nanoparticle caneach independently be a fluorescent nanoparticle. The first membermicrosphere can have a fluorescence emission distinguishable from afluorescence emission of the second member microsphere.

The first member microsphere can have a fluorescence wavelengthdistinguishable from a fluorescence wavelength of the second membermicrosphere. The first member microsphere can have a fluorescenceintensity distinguishable from a fluorescence intensity of the secondmember microsphere.

In another aspect, a method of tracking microspheres includes viewing afirst microsphere including a central region and a peripheral region ona surface of the central region, the peripheral region including a firstnanoparticle, and viewing a second microsphere including a centralregion and a peripheral region on a surface of the central region, theperipheral region including a second nanoparticle.

In the method, the first nanoparticle and the second nanoparticle caneach independently be a fluorescent nanoparticle. The first microspherecan have a fluorescence emission distinguishable from a fluorescenceemission of the second microsphere. The first microsphere can have asize distinguishable from a size of the second microsphere. Viewing thefirst microsphere can include observing a fluorescence emission from thefirst microsphere. The first microsphere can include a central regionincluding an inorganic material. The first microsphere can include aperipheral region including an inorganic material. The first microspherecan include a semiconductor nanocrystal.

The microspheres can be fluorescent particles with narrow sizedistributions and favorable fluorescence properties. Because themicrosphere cores and the semiconductor nanocrystals are synthesizedseparately, both can exhibit size monodispersity. Microspheres includingsemiconductor nanocrystals can have narrow emission linewidths. Thefluorescence properties of the microspheres can be more robust (e.g.,have greater chemical and photostability) than microspheresincorporating organic fluorescent dyes. Micrometer and sub-micrometersized fluorescent microspheres can be used in photonics and biologicalimaging.

The details of one or more embodiments are set forth in the accompanyingdrawings and the description below. Other features, objects, andadvantages will be apparent from the description and drawings, and fromthe claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a is a schematic depiction of a cross-section of a microsphereincorporating nanoparticles. FIG. 1 b is a schematic depiction of across-section of a semiconductor nanocrystal. FIG. 1 c is a schematicdepiction of a cross-section of a microsphere incorporatingnanoparticles.

FIG. 2 a is a schematic of the growth of a shell on a microsphere. FIG.2 b is a transmission electron microscope (TEM) image of microspheres.FIG. 2 c is a histogram depicting the size distribution of microspheres.

FIGS. 3 a–3 d are fluorescence micrographs of microspheres. The insetsof FIGS. 3 a–3 d are TEM images of the corresponding microspheres.

FIGS. 4 a and 4 b are photoluminescence spectra of microspheres.

FIG. 5 a is a TEM image of microspheres. FIG. 5 b is a fluorescencemicrograph of microspheres.

FIG. 6 is a fluorescence micrograph of microspheres in the blood vesselsof a mouse.

DETAILED DESCRIPTION

Microspheres containing chromophores have been utilized in an extensivevariety of applications, including photonic crystals, biologicallabeling, and flow visualization in microfluidic channels. See, forexample, Y. Lin, et al., Appl. Phys Lett. 2002, 81, 3134; D. Wang, etal., Chem. Mater. 2003, 15, 2724; X. Gao, et al., J. Biomed. Opt. 2002,7, 532; M. Han, et al., Nature Biotechnology. 2001, 19, 631; V. M. Pai,et al., Mag. & Magnetic Mater. 1999, 194, 262, each of which isincorporated by reference in its entirety. Both the photostability ofthe chromophores and the monodispersity of the microspheres can beimportant. Nanoparticles, such as, for example, metal nanoparticles,metal oxide nanoparticles, or semiconductor nanocrystals can beincorporated into microspheres. The optical, magnetic, and electronicproperties of the nanoparticles can allow them to be observed whileassociated with the microspheres and can allow the microspheres to beidentified and spatially monitored. For example, the highphotostability, good fluorescence efficiency and wide emissiontunability of colloidally synthesized semiconductor nanocrystals canmake them an excellent choice of chromophore. Unlike organic dyes,nanocrystals that emit different colors (i.e. different wavelengths) canbe excited simultaneously with a single light source. Colloidallysynthesized semiconductor nanocrystals (such as, for example, core-shellCdSe/ZnS and CdS/ZnS nanocrystals) can be incorporated intomicrospheres. The microspheres can be monodisperse silica microspheres.

The nanoparticle can be a metal nanoparticle, a metal oxidenanoparticle, or a semiconductor nanocrystal.

The metal nanoparticle or metal oxide nanoparticle can have a dimensionof less than 100 nanometers. The metal of the metal nanoparticle or themetal oxide nanoparticle can include titanium, zirconium, hafnium,vanadium, niobium, tantalum, chromium, molybdenum, tungsten, manganese,technetium, rhenium, iron, ruthenium, osmium, cobalt, rhodium, iridium,nickel, palladium, platinum, copper, silver, gold, zinc, cadmium,scandium, yttrium, lanthanum, a lanthanide series or actinide serieselement (e.g., cerium, praseodymium, neodymium, promethium, samarium,europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium,ytterbium, lutetium, thorium, protactinium, and uranium), boron,aluminum, gallium, indium, thallium, silicon, germanium, tin, lead,antimony, bismuth, polonium, magnesium, calcium, strontium, and barium.In certain embodiments, the metal can be iron, ruthenium, cobalt,rhodium, nickel, palladium, platinum, silver, gold, cerium or samarium.The metal oxide can be an oxide of any of these materials or combinationof materials. For example, the metal can be gold, or the metal oxide canbe an iron oxide (e.g., Fe₂O₃, Fe₃O₄), a cobalt oxide (e.g., CoO), azinc oxide (e.g., ZnO), a cerium oxide (e.g., CeO₂), or a titanium oxide(e.g., TiO₂). Preparation of metal and metal oxide nanoparticles isdescribed, for example, in U.S. Pat. Nos. 5,897,945 and 6,759,199, eachof which is incorporated by reference in its entirety.

Semiconductor nanocrystals with narrow size distributions and highluminescent efficiencies are an attractive alternative to organicmolecules in applications such as optoelectronic devices and biologicalfluorescence labeling. See, for example, V. L. Colvin, et al., Nature1994, 370, 354; B. O. Dabbousi, et al., Appl. Phys. Lett. 1995, 66,1316; M. Bruchez Jr., et al., Science 1998, 281, 2013; W. C. W. Chan,and S. Nie, Science 1998, 281, 2016; and H. Mattoussi, et al., J. Am.Chem. Soc. 2000, 122, 12142, each of which is incorporated by referencein its entirety. Semiconductor nanocrystals can be more stable tophotobleaching and have a more saturated fluorescence (i.e., narroweremission bandwidths) compared to organic molecules. Their opticalproperties are size-tunable and independent of their chemicalproperties.

The method of manufacturing a nanocrystal is a colloidal growth process.See, for example, U.S. Pat. Nos. 6,322,901 and 6,576,291, each of whichis incorporated by reference in its entirety. Colloidal growth occurs byrapidly injecting an M-containing compound and an X donor into a hotcoordinating solvent. The coordinating solvent can include an amine. TheM-containing compound can be a metal, an M-containing salt, or anM-containing organometallic compound. The injection produces a nucleusthat can be grown in a controlled manner to form a nanocrystal. Thereaction mixture can be gently heated to grow and anneal thenanocrystal. Both the average size and the size distribution of thenanocrystals in a sample are dependent on the growth temperature. Thegrowth temperature necessary to maintain steady growth increases withincreasing average crystal size. The nanocrystal is a member of apopulation of nanocrystals. As a result of the discrete nucleation andcontrolled growth, the population of nanocrystals obtained has a narrow,monodisperse distribution of diameters. The monodisperse distribution ofdiameters can also be referred to as a size. The process of controlledgrowth and annealing of the nanocrystals in the coordinating solventthat follows nucleation can also result in uniform surfacederivatization and regular core structures. As the size distributionsharpens, the temperature can be raised to maintain steady growth. Byadding more M-containing compound or X donor, the growth period can beshortened.

The M-containing salt can be a non-organometallic compound, e.g., acompound free of metal-carbon bonds. M is cadmium, zinc, magnesium,mercury, aluminum, gallium, indium, thallium, or lead. The M-containingsalt can be a metal halide, metal carboxylate, metal carbonate, metalhydroxide, metal oxide, or metal diketonate, such as a metalacetylacetonate. The M-containing salt is less expensive and safer touse than organometallic compounds, such as metal alkyls. For example,the M-containing salts are stable in air, whereas metal alkyls aregenerally unstable in air. M-containing salts such as 2,4-pentanedionate(i.e., acetylacetonate (acac)), halide, carboxylate, hydroxide, oxide,or carbonate salts are stable in air and allow nanocrystals to bemanufactured under less rigorous conditions than corresponding metalalkyls.

Suitable M-containing salts include cadmium acetylacetonate, cadmiumiodide, cadmium bromide, cadmium chloride, cadmium hydroxide, cadmiumcarbonate, cadmium acetate, cadmium oxide, zinc acetylacetonate, zinciodide, zinc bromide, zinc chloride, zinc hydroxide, zinc carbonate,zinc acetate, zinc oxide, magnesium acetylacetonate, magnesium iodide,magnesium bromide, magnesium chloride, magnesium hydroxide, magnesiumcarbonate, magnesium acetate, magnesium oxide, mercury acetylacetonate,mercury iodide, mercury bromide, mercury chloride, mercury hydroxide,mercury carbonate, mercury acetate, aluminum acetylacetonate, aluminumiodide, aluminum bromide, aluminum chloride, aluminum hydroxide,aluminum carbonate, aluminum acetate, gallium acetylacetonate, galliumiodide, gallium bromide, gallium chloride, gallium hydroxide, galliumcarbonate, gallium acetate, indium acetylacetonate, indium iodide,indium bromide, indium chloride, indium hydroxide, indium carbonate,indium acetate, thallium acetylacetonate, thallium iodide, thalliumbromide, thallium chloride, thallium hydroxide, thallium carbonate, orthallium acetate.

Alkyl is a branched or unbranched saturated hydrocarbon group of 1 to100 carbon atoms, preferably 1 to 30 carbon atoms, such as methyl,ethyl, n-propyl, isopropyl, n-butyl, isobutyl, t-butyl, octyl, decyl,tetradecyl, hexadecyl, eicosyl, tetracosyl and the like, as well ascycloalkyl groups such as cyclopentyl, cyclohexyl and the like.Optionally, an alkyl can contain 1 to 6 linkages selected from the groupconsisting of —O—, —S—, -M- and —NR— where R is hydrogen, or C₁–C₈ alkylor lower alkenyl.

Prior to combining the M-containing salt with the X donor, theM-containing salt can be contacted with a coordinating solvent to forman M-containing precursor. Typical coordinating solvents include alkylphosphines, alkyl phosphine oxides, alkyl phosphonic acids, or alkylphosphinic acids; however, other coordinating solvents, such aspyridines, furans, and amines may also be suitable for the nanocrystalproduction. Examples of suitable coordinating solvents include pyridine,tri-n-octyl phosphine (TOP) and tri-n-octyl phosphine oxide (TOPO).Technical grade TOPO can be used. The coordinating solvent can include a1,2-diol or an aldehyde. The 1,2-diol or aldehyde can facilitatereaction between the M-containing salt and the X donor and improve thegrowth process and the quality of the nanocrystal obtained in theprocess. The 1,2-diol or aldehyde can be a C₆–C₂₀ 1,2-diol or a C₆–C₂₀aldehyde. A suitable 1,2-diol is 1,2-hexadecanediol and a suitablealdehyde is dodecanal.

The X donor is a compound capable of reacting with the M-containing saltto form a material with the general formula MX. Typically, the X donoris a chalcogenide donor or a pnictide donor, such as a phosphinechalcogenide, a bis(silyl) chalcogenide, dioxygen, an ammonium salt, ora tris(silyl) pnictide. Suitable X donors include dioxygen, elementalsulfur, bis(trimethylsilyl) selenide ((TMS)₂Se), trialkyl phosphineselenides such as (tri-n-octylphosphine) selenide (TOPSe) or(tri-n-butylphosphine) selenide (TBPSe), trialkyl phosphine telluridessuch as (tri-n-octylphosphine) telluride (TOPTe) orhexapropylphosphorustriamide telluride (HPPTTe),bis(trimethylsilyl)telluride ((TMS)₂Te), sulfur,bis(trimethylsilyl)sulfide ((TMS)₂S), a trialkyl phosphine sulfide suchas (tri-n-octylphosphine) sulfide (TOPS), tris(dimethylamino) arsine, anammonium salt such as an ammonium halide (e.g., NH₄Cl),tris(trimethylsilyl) phosphide ((TMS)₃P), tris(trimethylsilyl) arsenide((TMS)₃As), or tris(trimethylsilyl) antimonide ((TMS)₃Sb). In certainembodiments, the M donor and the X donor can be moieties within the samemolecule.

The nanocrystal manufactured from an M-containing salt grows in acontrolled manner when the coordinating solvent includes an amine. Theamine in the coordinating solvent can contribute to the quality of thenanocrystal obtained from the M-containing salt and X donor. Preferably,the coordinating solvent is a mixture of the amine and an alkylphosphine oxide in a mole ratio of 10:90, more preferably 30:70 and mostpreferably 50:50. The combined solvent can decrease size dispersion andcan improve photoluminescence quantum yield of the nanocrystal. Thepreferred amine is a primary alkyl amine or a primary alkenyl amine,such as a C₂–C₂₀ alkyl amine, a C₂–C₂₀ alkenyl amine, preferably aC₈–C₁₈ alkyl amine or a C₈–C₁₈ alkenyl amine. For example, suitableamines for combining with tri-octylphosphine oxide (TOPO) include1-hexadecylamine, or oleylamine. When the 1,2-diol or aldehyde and theamine are used in combination with the M-containing salt to form apopulation of nanocrystals, the photoluminescence quantum efficiency andthe distribution of nanocrystal sizes are improved in comparison tonanocrystals manufactured without the 1,2-diol or aldehyde or the amine.

The nanocrystal can be a member of a population of nanocrystals having anarrow size distribution. The nanocrystal can be a sphere, rod, disk, orother shape. The nanocrystal can include a core of a semiconductormaterial. The nanocrystal can include a core having the formula MX,where M is cadmium, zinc, magnesium, mercury, aluminum, gallium, indium,thallium, or mixtures thereof, and X is oxygen, sulfur, selenium,tellurium, nitrogen, phosphorus, arsenic, antimony, or mixtures thereof.

The emission from the nanocrystal can be a narrow Gaussian emission bandthat can be tuned through the complete wavelength range of theultraviolet, visible, or infrared regions of the spectrum by varying thesize of the nanocrystal, the composition of the nanocrystal, or both.For example, both CdSe and CdS can be tuned in the visible region andInAs can be tuned in the infrared region.

A population of nanocrystals can have a narrow size distribution. Thepopulation can be monodisperse and can exhibit less than a 15% rmsdeviation in diameter of the nanocrystals, preferably less than 10%,more preferably less than 5%. Spectral emissions in a narrow range ofbetween 10 and 100 nm full width at half max (FWHM) can be observed.Semiconductor nanocrystals can have emission quantum efficiencies ofgreater than 2%, 5%, 10%, 20%, 40%, 60%, 70%, or 80%.

The semiconductor forming the core of the nanocrystal can include GroupII-VI compounds, Group II-V compounds, Group III-VI compounds, GroupIII-V compounds, Group IV-VI compounds, Group I-III-VI compounds, GroupII-IV-VI compounds, and Group II-IV-V compounds, for example, ZnS, ZnSe,ZnTe, CdS, CdSe, CdTe, HgS, HgSe, HgTe, AlN, AlP, AlAs, AlSb, GaN, GaP,GaAs, GaSb, GaSe, InN, InP, InAs, InSb, TlN, TlP, TlAs, TlSb, PbS, PbSe,PbTe, or mixtures thereof.

The quantum efficiency of emission from nanocrystals having a core of afirst semiconductor material can be enhanced by applying an overcoatingof a second semiconductor material such that the conduction band of thesecond semiconductor material is of higher energy than that of the firstsemiconductor material, and the valence band of the second semiconductormaterial is of lower energy than that of the first semiconductormaterial. As a result, charge carriers, i.e., electrons and holes, areconfined in the core of the nanocrystal when in an excited state.Alternatively, the conduction band or valence band of overcoatingmaterial can have an energy intermediate between the energies of theconduction and valence bands of the core material. In this case, onecarrier can be confined to the core while the other is confined to theovercoating material when in an excited state. See, for example, U.S.patent application Ser. No. 10/638,546, which is incorporated byreference in its entirety. The core can have an overcoating on a surfaceof the core. The overcoating can be a semiconductor material having acomposition different from the composition of the core, and can have aband gap greater than the band gap of the core. The overcoat of asemiconductor material on a surface of the nanocrystal can include aGroup II-VI compounds, Group II-V compounds, Group III-VI compounds,Group III-V compounds, Group IV-VI compounds, Group I-III-VI compounds,Group II-IV-VI compounds, and Group II-IV-V compounds, for example, ZnS,ZnSe, ZnTe, CdS, CdSe, CdTe, HgS, HgSe, HgTe, AlN, AlP, AlAs, AlSb, GaN,GaP, GaAs, GaSb, GaSe, InN, InP, InAs, InSb, TlN, TlP, TlAs, TlSb, PbS,PbSe, PbTe, or mixtures thereof.

The outer surface of the nanocrystal can include a layer of compoundsderived from the coordinating agent used during the growth process. Thesurface can be modified by repeated exposure to an excess of a competingcoordinating group to form an overlayer. For example, a dispersion ofthe capped nanocrystal can be treated with a coordinating organiccompound, such as pyridine, to produce crystals which disperse readilyin pyridine, methanol, and aromatics but no longer disperse in aliphaticsolvents. Such a surface exchange process can be carried out with anycompound capable of coordinating to or bonding with the outer surface ofthe nanocrystal, including, for example, phosphines, thiols, amines andphosphates. The nanocrystal can be exposed to short chain polymers whichexhibit an affinity for the surface and which terminate in a moietyhaving an affinity for a suspension or dispersion medium. Such affinityimproves the stability of the suspension and discourages flocculation ofthe nanocrystal.

Monodentate alkyl phosphines (and phosphine oxides; the term phosphinebelow will refer to both) can passivate nanocrystals efficiently. Whennanocrystals with conventional monodentate ligands are diluted orembedded in a non-passivating environment (i.e., one where no excessligands are present), they tend to lose their high luminescence. Typicalare an abrupt decay of luminescence, aggregation, and/or phaseseparation. In order to overcome these limitations, polydentate ligandscan be used, such as a family of polydentate oligomerized phosphineligands. The polydentate ligands show a high affinity between ligand andnanocrystal surface. In other words, they are stronger ligands, as isexpected from the chelate effect of their polydentate characteristics.

Oligomeric phosphines have more than one binding site to the nanocrystalsurface, which ensures their high affinity to the nanocrystal surface.See, for example, for example, U.S. Ser. No. 10/641,292, filed Aug. 15,2003, and U.S. Ser. No. 60/403,367, filed Aug. 15, 2002, each of whichis incorporated by reference in its entirety. The oligomeric phosphinecan be formed from a monomeric, polyfunctional phosphine, such as, forexample, tris(hydroxypropyl)phosphine, and a polyfunctionaloligomerization reagent, such as, for example, a diisocyanate. Theoligomeric phosphine can be contacted with an isocyanate of

formula R′-L-NCO, wherein L is C₂–C₂₄ alkylene, and R′ has the formula

R′ has the formula

or R′ is hydrogen, wherein R^(a) is hydrogen or C₁–C₄ alkyl.

An overcoating process is described, for example, in U.S. Pat. No.6,322,901, incorporated herein by reference in its entirety. Byadjusting the temperature of the reaction mixture during overcoating andmonitoring the absorption spectrum of the core, overcoated materialshaving high emission quantum efficiencies and narrow size distributionscan be obtained. Alternatively, an overcoating can be formed by exposinga core nanocrystal having a first composition and first average diameterto a population of nanocrystals having a second composition and a secondaverage diameter smaller than the first average diameter.

Size distribution during the growth stage of the reaction can beestimated by monitoring the absorption line widths of the particles.Modification of the reaction temperature in response to changes in theabsorption spectrum of the particles allows the maintenance of a narrowparticle size distribution during growth. Reactants can be added to thenucleation solution during crystal growth to grow larger crystals. Bystopping growth at a particular nanocrystal average diameter, apopulation having an average nanocrystal diameter of less than 150 Å canbe obtained. A population of nanocrystals can have an average diameterof 15 Å to 125 Å. The emission spectra of the nanocrystals can be tunedcontinuously over the wavelength range of 300 nm to 5 microns, or forexample, when CdSe or CdTe is the core material, from 400 nm to 800 nm.IR-emitting semiconductor nanocrystals can be prepared according tomethods described in, for example, U.S. patent application Ser. No.10/638,546, which is incorporated by reference in its entirety.

The particle size distribution can be further refined by size selectiveprecipitation with a poor solvent for the nanocrystals, such asmethanol/butanol as described in U.S. Pat. No. 6,322,901, incorporatedherein by reference in its entirety. For example, nanocrystals can bedispersed in a solution of 10% butanol in hexane. Methanol can be addeddropwise to this stirring solution until opalescence persists.Separation of supernatant and flocculate by centrifugation produces aprecipitate enriched with the largest crystals in the sample. Thisprocedure can be repeated until no further sharpening of the opticalabsorption spectrum is noted. Size-selective precipitation can becarried out in a variety of solvent/nonsolvent pairs, includingpyridine/hexane and chloroform/methanol. The size-selected nanocrystalpopulation can have no more than a 15% rms deviation from mean diameter,preferably 10% rms deviation or less, and more preferably 5% rmsdeviation or less.

Transmission electron microscopy (TEM) can provide information about thesize, shape, and distribution of the nanocrystal population. PowderX-ray diffraction (XRD) patterns can provide the most completeinformation regarding the type and quality of the crystal structure ofthe nanocrystals. Estimates of size are also possible since particlediameter is inversely related, via the X-ray coherence length, to thepeak width. For example, the diameter of the nanocrystal can be measureddirectly by transmission electron microscopy or estimated from X-raydiffraction data using, for example, the Scherrer equation. It also canbe estimated from the UVN is absorption spectrum, if calibrated by adirect measurement of diameter, such as XRD or TEM.

Fluorescent semiconductor nanocrystals have been introduced intomicrospheres by either synthesizing the nanocrystals in situ using themicrosphere as a host matrix, or using mercaptosiloxane functionalizednanocrystals as “seeds” for growing microspheres. See, for example, Y.Lin, et al. Appl. Phys Lett. 2002, 81, 3134; N. A. Dhas, et al. Chem.Mater. 1999, 11, 806; M. A. Correa-Duarte, et al., Chem. Phys. Lett.1998, 286, 497; A. L. Rogach, et al., Chem. Mater. 2000, 12, 2676; andJ. N. Cha, et al., Nano Lett. 2003, 3, 907, each of which isincorporated by reference in its entirety. These methods, however,produced either relatively low quality nanocrystals or microspheres ofsignificant polydispersity. Significantly broadened and deep trapemission can also result from the silication process. Relativelymonodisperse mesoporous polystyrene and silica microspheres wereimpregnated with high quality CdSe/ZnS core-shell nanocrystals (see, forexample, X. Gao, S. Nie, Anal. Chem. 2004, 76, 2406; and X. Gao, S. Nie,J. Phys. Chem. B 2003, 107, 11575, each of which is incorporated byreference in its entirety). In the case of polystyrene, this wasachieved through hydrophobic interactions between polystyrene and thehydrophobic ligands on the nanocrystal surface. For silica the methodwas similar, except the surface pores of the silica microspheres werefirst coated with small hydrophobic molecules. Because the nanocrystalswere not chemically bound to the internal surface of the sphere,exposure to non-polar solvents caused the nanocrystals to leach out.This would be unsuitable in a flow visualization experiment, forexample, where the velocity profiles of fluorescent microspheres in anon-polar solvent are to be determined. The pore sizes of the mesoporousmicrospheres were on the order of tens of nanometers. This set a lowerlimit of approximately 1 micrometer to possible microsphere sizes.

Semiconductor nanocrystals can be incorporated into a silica or titaniashell, or coat, grown on preformed sub-micrometer diameter silicamicrospheres. The coated microspheres can be dispersed in a variety ofpolar and non-polar solvents. The microspheres can be robust. Forexample, repeated sonication and washes in solvents does not result inany evident loss of nanocrystals.

Referring to FIG. 1 a, microsphere 100 includes central particle 110 andperipheral layer 120. Central particle 110 can be a silica microsphere.Peripheral layer 120 can include an inorganic material, such as silicaor titania. Dispersed in peripheral layer 120 are nanoparticles 130.Nanoparticles 130 can include a metal nanoparticle, for example, a goldnanoparticle, a magnetic nanoparticle, for example, a cobalt or ironoxide nanocrystal, or a semiconductor nanocrystal. The nanoparticles 130dispersed in peripheral layer 120 can be alike or different. Forexample, microsphere 100 can include a mixture of magnetic cobaltnanoparticles and semiconductor nanocrystals. In another example,microsphere 100 can include semiconductor nanocrystals of differentsizes or compositions. FIG. 1 b shows a cross section of a semiconductornanocrystal 140, including a core 150, optionally shell 160, and outerlayer 170. Core 150 includes a first semiconductor material. Shell 160can include a second semiconductor material. Outer layer 170 can includea ligand that has an affinity for a surface of semiconductor nanocrystal140.

A nanoparticle 130 can include a ligand that provides solubility in adesired medium, such as a non-polar solvent, or a polar solvent, such asethanol or water. The ligand can have one end that has affinity for thesurface of nanoparticle 130, and another that is capable of reactingwith a precursor to a peripheral layer. The precursor can be a polymerprecursor. The precursor can be, for example, a silica precursor, suchas tetraethoxysilane, or a titania precursor, such astetrabutylorthotitanate. The end of the ligand that reacts with theprecursor can become incorporated in the peripheral layer. In this waythe nanoparticle can become anchored to the peripheral layer. The ligandcan become covalently bound to the peripheral layer material.

Central particle 110 can have a diameter of less than 1 millimeter, suchas, for example, less than 500 micrometers, less than 100 micrometers,less than 10 micrometers, or less than 1 micrometer. For example,central particle 110 can have a diameter between 100 nm and 1000 nm.Central particle 110 can be a member of a monodisperse population ofparticles. For example, central particle 110 can be a member of amonodisperse population of microspheres. When central particle 110 is amember of a monodisperse population of microspheres, peripheral layers120 can be formed individually on central particles 110 withoutsubstantially altering the monodispersity of the microsphere population.In other words, a population of microspheres 100 formed from amonodisperse population of particles 110 can be a monodispersepopulation of microspheres.

As shown in FIG. 1 c, microsphere 200 includes central particle 210 anda first peripheral layer 220. Dispersed in peripheral layer 220 arenanoparticles 230. Microsphere 200 also includes a second peripherallayer 240. Dispersed in peripheral layer 240 are nanoparticles 250.Microsphere 200 can be made by forming a second peripheral layer on amicrosphere 100. The nanoparticles 230 in the first peripheral layer 220can have the same composition or a different composition as thenanoparticles 250 in the second peripheral layer 240. The nanoparticlesin the first peripheral layer 220 can have the same size or a differentsize as the nanoparticles in the second peripheral layer 240. Forexample, nanoparticles 230 can be gold nanoparticles and nanoparticles250 can be magnetic cobalt nanoparticles. In another example,nanoparticles 230 can be CdSe/ZnS semiconductor nanocrystals andnanoparticles 250 can be CdS/ZnS semiconductor nanocrystals. In anotherexample, nanoparticles 230 are CdSe/ZnS semiconductor nanocrystalshaving a different average diameter from nanoparticles 250, which arealso CdSe/ZnS semiconductor nanocrystals. Additional peripheral layerscan be added. The additional peripheral layers can similarly includenanoparticles.

In general, a single microsphere can include nanoparticles of a singlesize and composition. Alternatively, the microsphere can includenanoparticles of more than one size, and having the same composition; orthe nanoparticles can have different sizes and compositions. Thus, amicrosphere can be prepared that fluoresces at a single wavelength or atmultiple wavelengths; is both fluorescent and magnetic; or has othercombinations of properties derived from the nanoparticles.

Microspheres incorporating fluorescent materials can be distinguished bythe color (i.e. wavelength) of fluorescence emission, the intensity ofthe emission, or both. For example, two microspheres can incorporatematerials that fluoresce at different wavelengths (i.e. the emissionwavelengths can be distinguished); or can incorporate materials thatfluoresce at the same wavelength but with distinguishable intensities.The fluorescence properties can be correlated with a size ofmicrosphere, such that a population of microspheres having a narrow sizedistribution shares fluorescence wavelength (or wavelengths) andintensity (or intensities). When a size measurement is inconvenient orimpractical, the fluorescence properties can be used to distinguish,identify or track microspheres of a particular size. See, for example,U.S. Pat. No. 6,617,583, which is incorporated by reference in itsentirety.

Microspheres can be formed from a metal, ceramic, or polymer. Themicrosphere can include an inorganic material or an inorganic material.Some examples of microsphere materials are silica, titania,poly(divinylbenzene), poly(styrene), and poly(methylmethacrylate). Acoating can be grown on a surface of a microsphere. For example, thecoating can include silica or titania. A nanoparticle, such as asemiconductor nanocrystal, can be incorporated in the coating.

A shell of silica can be grown on a silica microsphere in the presenceof properly derivatized nanoparticles, such as semiconductornanocrystals. See, for example, A. van Blaaderen, and A. Vrij, Langmuir1992, 8, 2921, which is incorporated by reference in its entirety. Thecore silica microspheres can be synthesized using previously establishedtechniques (see, for example, G. H. Bogush, et al., J. Non-Cryst. Solids1988, 104, 95; and W. Stöber, et al., J. Colloid Interface Sci., 1968,26, 62, each of which is incorporated by reference in its entirety). Toincorporate the nanocrystals into the microsphere shell, the ligands onthe nanocrystal can impart both ethanol solubility and chemicalcompatibility with the silica matrix. The ligands on the nanocrystalsurface (for example, TOPO ligands) can be exchanged for a ligand whichpromotes ethanol solubility and a ligand which can react with silicaprecursors. For example, the ligand can be an alkoxy silane havingmoiety including a hydroxy, sulfhydryl, carboxy, disulfide, phosphine,phosphite, or amino group.

For example, TOPO can be exchanged for a mixture of 5-amino-1-pentanol(AP), which can promote ethanol solubility, and3-aminopropyltrimethoxysilane (APS), which can react with a silicaprecursor. Other ligands that promote ethanol solubility or silicaprecursor reactivity can be used. The amino groups of AP and APS canbind to the nanocrystal surface. The hydroxyl group of AP can permitdispersion in ethanol while the alkoxysilane moiety of APS can allow theformation of siloxane bonds with the silica host matrix. The properlycap-exchanged nanocrystals can be dispersed in a mixture of ethanol, asilica precursor (such as tetraethoxysilane) and silica microspheres.Addition of water and ammonium hydroxide to this mixture at elevatedtemperatures causes rapid hydrolysis of the siloxane precursor, whichsubsequently condenses to form a thin shell of silica around themicrosphere. See FIG. 2 a.

FIG. 2 b shows a TEM image of a distribution of silica microspheres withCdSe/ZnS nanocrystals localized in the peripheral regions of themicrospheres. The inset of FIG. 2 b shows a microsphere from the samedistribution imaged at a higher magnification to highlight the smoothmorphology of these silica/silica-nanocrystal microspheres. The sizedispersities of samples were quantified. The diameters of ˜200 sphereswere obtained from scanned TEM images processed using Image J (Version1.30, July 2003, Wayne Rasband, National Institutes of Health) andtabulated. The size dispersity (the standard deviation divided by themean sphere diameter) was subsequently evaluated as such for all samplesreported. FIG. 2 c shows the size dispersity of the sample from FIG. 2b, along with that of the starting microspheres, illustrating that thesize dispersity of microspheres does not significantly change upongrowth of a peripheral layer. The thickness of the peripheral layercannot be determined by simply taking the difference in mean diametersbetween the microspheres before and after peripheral layer growth,because the microspheres can shrink as a result of condensation ofunreacted Si—OH groups when they are re-dispersed in a basic solutionfor overcoating. See, e.g., C. J. Brinker, and G. W. Scherer, “Sol-gelScience—The Physics and Chemistry of Sol-gel Processing”, AcademicPress, Boston 1990, which is incorporated by reference in its entirety.A range of concentrations of nanocrystals can be used (such as 1×10⁻⁴mmol/L to 8×10⁻⁴ mmol/L) without perturbing the peripheral layer growthprocess, but it is likely that extremely high concentrations ofnanocrystals can result in agglomeration of nanocrystals andcrosslinking of spheres due to the large excess of APS in solution. Theperipheral layer growth process can allow the incorporation of differentcolor-emitting nanocrystals into silica microspheres. Silicamicrospheres can have diameters ranging from, e.g., 100 nanometers to500 micrometers, 150 nanometers to 100 micrometers, 150 nanometers to 10micrometers, or 154 nanometers to 954 nanometers after overcoating. SeeFIG. 3, which displays fluorescence micrographs of (a) 154 nm (±6.6%)diameter microspheres with nanocrystals emitting at 604 nm; (b) 295 nm(±4.1%) diameter microspheres with nanocrystals emitting at 580 nm; (c)609 nm (±2.5%) diameter microspheres with nanocrystals emitting at 531nm; and (d) 954 nm (±2.7%) diameter microspheres with nanocrystalsemitting at 625 nm. The insets of FIGS. 3 a–3 d are TEM images of themicrospheres. A peripheral layer can be grown on commercially availablesilica microspheres by the method. The peripheral layer growthconditions can be fine tuned with respect to the size of the microsphereso that the nanocrystals are introduced under silica shell growthconditions, rather than silica nucleation conditions. The slowerkinetics of silica growth permits greater control of nanocrystalincorporation and preserves the size monodispersity of the finalmicrosphere composites. The emission spectra of microspheresincorporating semiconductor nanocrystals are shown in FIG. 4 a. Themicrospheres were dispersed in an index-matched liquid. FIG. 4 b, whichpresents emission spectra of 3.8 nm CdSe/ZnS nanocrystals in hexanebefore incorporation (peak at 578 nm) and incorporated into microspheressuspended in an index-matching liquid after overcoating (peak at 580nm). The incorporation process did not result in significant broadeningor any deep trap emission from the nanocrystals, unlike the resultsreported in M. A. Correa-Duarte, et al., Chem. Phys. Lett. 1998, 286,497, which is incorporated by reference in its entirety.

The quantity of nanocrystals incorporated into the silica peripherallayer was estimated by first acquiring the absorption spectrum of aknown weight of overcoated microspheres. This yielded the total numberof nanocrystals and microspheres. See, for example, C. A. Leatherdale,et al., J. Phys. Chem. B 2002, 106, 7619, which is incorporated byreference in its entirety. The microspheres were immersed in arefractive index matching liquid (a mixture of ethanol, n=1.36, andtoluene, n=1.49), in order to minimize effects from light scattering bythe microspheres. The reported density of Stöber silica microspheres is2.03 g/cm³, with a corresponding index of refraction of 1.46 (see, forexample, A. van Blaaderen, and A. Vrij, Langmuir 1992, 8, 2921). Thisagreed with the refractive index determined for the microspheres afterperipheral layer growth. Use of this reported density allowedquantification of the number of nanocrystals per microsphere. As anexample, the 295 nm microspheres in FIG. 3 b contained ˜1200nanocrystals per microsphere (˜0.3% volume fraction). Knowing the numberdensity of nanocrystals in spheres also enabled determination of thequantum yield within the sphere, and quantum yields as high as 13% wereobtained.

The quantum yield was determined as follows: the optical densities ofthe index-matched spheres in a toluene/ethanol mixture and anappropriate reference dye in methanol were closely matched at thewavelength of excitation. To ensure that no reabsorption of the dyeemission occurs, the optical densities were always maintained at a valuebelow 0.1 at the excitation wavelength. The photoluminescence spectra ofboth the sample of spheres and the reference dye were acquired using aSPEX Fluorolog 1680 spectrometer. Comparison of their correspondingintegrated emission allowed the quantum yield of the sample to bedetermined. Although the quantum yields of as-synthesized core-shellCdSe/ZnS nanocrystals used were as high as 38%, subsequent loss of theoriginal surface ligands due to cap-exchange with AP and APS can lead todiminished quantum yields. Furthermore, the decline in the quantum yielddue to processing is very dependent on the quality and thickness of theZnS shell on the nanocrystals, which can vary from sample to sample.

The photostability of the nanocrystals (˜3.8 nm diameter) in the silicamicrospheres was evaluated using a 514 nm excitation source from a CWAr⁺laser focused through a 0.95 NA air objective at an intensity of 80W/cm². No appreciable decrease in the fluorescence intensity was seenover a period of 20 minutes, suggesting that little, if any,photobleaching occurred.

Although the spatial distribution of the nanocrystals within the silicashell could not easily be imaged directly via TEM, WDS measurements on anumber of randomly chosen individual microspheres indicate a uniformdistribution of nanocrystals from sphere to sphere, with an average Seto Si elemental mass ratio of 0.026±13% (the standard deviation is moresignificant than the absolute value of the ratio due to the curvature ofthe microsphere, which may introduce inherent systematic error into theWDS measurement). This result suggested that the nanocrystals are notaggregated, and are likely evenly distributed throughout the shell. Theslight 2 nm red shift in the emission spectrum of incorporatednanocrystals, as seen in FIG. 4 b, is further evidence fornon-aggregation of nanocrystals. Moreover, characterization ofmicrospheres using high resolution TEM revealed a relatively smoothsurface morphology with no apparent nanocrystals localized on thesurface, consistent with the nanocrystals not existing in an aggregatedform inside the microsphere.

An amorphous titania peripheral layer including CdSe/ZnS core-shellnanocrystals was grown on silica microspheres. The procedure closelyresembles that for growing a silica peripheral layer on microspheres,with slight modifications as described below. Fluorescence microscopyand TEM images of silica microspheres with a titania/nanocrystalperipheral layer are provided in FIG. 5. Unlike the silica peripherallayer silica microspheres, the difference in composition between thetitania peripheral layer and the silica particle gives enough contraston the TEM image to directly determine the thickness of the peripherallayer. The peripheral layer thickness was ˜60 nm. The refractive indexcontrast between the nanocrystal-doped titania peripheral layer and aircan permit use of the microspheres as photonic materials. WDSmeasurements confirmed the uniformity of the distribution ofnanocrystals from sphere to sphere, with a Se to Ti elemental mass ratioof 0.019±11%.

EXAMPLES

Silica Peripheral Layer Growth on Microspheres

CdSe and CdS nanocrystals were synthesized and then overcoated with ZnSusing previously reported techniques. See, for example, B. R. Fischer,et al., J. Phys. Chem. B 2004, 108, 143; J. S. Steckel, et al., Angew.Chem. Int. Ed. 2004, 43, 2154; and B. O. Dabbousi, et al., J. Phys.Chem. B 1997, 101, 9463, each of which is incorporated by reference inits entirety. The resulting core-shell nanocrystals dispersed inbutanol/hexane were then precipitated from solution using excessmethanol. Repeating this dispersion/precipitation cycle about threetimes removes most of the native trioctylphosphine (TOPO) caps, leavinga powder which was dried under vacuum and brought into anitrogen-atmosphere box for subsequent cap-exchange with AP and APS. Asan example, ˜135 mg of CdSe/ZnS nanocrystals (about 4.0 nm in diameter)was mixed with 1000 mg of anhydrous ethanol and 150 mg of APS, forming asuspension of nanocrystals. Addition of 60 mg of AP resulted in a clearsolution which was subsequently heated at ˜40° C. for about 30 minutesto ensure cap-exchange with AP and APS. Addition of 10 μL of thisnanocrystal solution to 30 mg bare silica microspheres (see below) and16 mg hydroxypropyl cellulose (avg. M_(w)=370,000) dispersed in 10 mL ofvigorously stirring ethanol was followed by the addition of 50 μL H₂O,50 μL NH₄OH (28% in H₂O) and 0.15 mL tetraethoxysilane. The mixture wasthen immersed in an oil bath at 75° C. for 4 hours while stirring. Thesilica/silica-nanocrystal microspheres were then isolated by 3–4 cyclesof centrifugation to precipitate followed by redispersion in ethanol.

Titania Peripheral Layer Growth on Microspheres

Addition of 10 μL of the nanocrystal solution in ethanol described aboveto 30 mg bare silica microspheres (see below) and 16 mg hydroxypropylcellulose in 10 mL of stirring ethanol was followed by the addition of54 μL H₂O and 0.1 mL tetrabutylorthotitanate. The mixture was thenimmersed in an oil bath at 75° C. for 4 hours while maintainingstirring. The silica/titania-nanocrystal microspheres were then isolatedby 3–4 cycles of centrifugation to precipitate followed by re-dispersalin ethanol.

Silica Microspheres

Various sizes (˜130–900 nm in diameter) were synthesized by variants ofthe well known Stöber process. See, for example, G. H. Bogush, et al.,J. Non-Cryst. Solids 1988, 104, 95; and W. Stöber, et al., J. ColloidInterface Sci., 1968, 26, 62, each of which is incorporated by referencein its entirety. For 130 nm spheres, for example, into a solution of0.55 mL tetraethoxysilane in 10 mL ethanol, 1 mL H₂O and 0.2 mL NH₄OH(28% in H₂O) were rapidly injected under vigorous stirring. Stirring wascontinued for 3 or more hours. The resulting microspheres were thenisolated from excess reagents by 3 cycles of centrifugation followed byre-dispersal in ethanol.

In Vivo Imaging

The microspheres can be used for in vivo imaging. Microspheres of twodistinct diameters were labeled with semiconductor nanocrystals ofdifferent emission wavelengths and administered via carotid arteryinjection to a mouse bearing a cranial window and expressing the greenfluorescent protein (GFP) in vascular endothelial cells (Tie2-GFPmouse). See, for example, R. J. Melder, et al., Microvasc. Res. 1995,50, 35; and T. Motoike, et al., Genesis 2000, 28, 75, each of which isincorporated by reference in its entirety. The microspheres were coatedwith polyethylene glycol (PEG) to increase residence times in the bloodvessels. Circulating microspheres were imaged by using multiphotonmicroscopy (MPM) using 800 nm light delivered through a 20X, 0.9 NAwater-immersion lens (see, for example, E. B. Brown, et al., Nat. Med.2001, 7, 864, which is incorporated by reference in its entirety).Circulating microspheres could be tracked using MPM intravitally. SeeFIG. 6. The ability to track distinct microspheres of multiplewell-defined sizes and colors simultaneously provides crucialinformation regarding flow characteristics in blood vessels, which canin turn guide drug delivery strategies (see R. K. Jain, Nat. Med. 1998,4, 655, which is incorporated by reference in its entirety). Themonodispersity of the microspheres allows for similar sized spheres(diameters of 400 nm and 500 nm, for example) having different emissionwavelengths to be utilized and reliably distinguished in biologicalexperiments in which sub-micron sizes are important. See, e.g., S. K.Hobbs, et al., Proc. Natl. Acad. Sci. 1998, 95, 4607; and S. L. Hale, etal., Am. J. Physiol. Heart. Circ. Physiol. 1986, 251, H863, each ofwhich is incorporated by reference in its entirety. Moreover, thefavorable optical properties of high-quality semiconductor nanocrystals,in particular the simultaneous excitation of different-colorednanocrystals, narrow emission profiles, and high photostability, can beharnessed for biological imaging applications inaccessible to organicdye-doped submicron spheres.

Other embodiments are within the scope of the following claims.

1. A microsphere comprising: a central region including a first matrixmaterial; and a first peripheral layer on a surface of the centralregion, the first peripheral layer including a second matrix materialdifferent from the first matrix material and including a firstnanoparticle dispersed within the layer, wherein the first nanoparticleis covalently linked to the first peripheral layer.
 2. The microsphereof claim 1, wherein the central region includes an inorganic material.3. The microsphere of claim 2, wherein the central region includessilicon.
 4. The microsphere of claim 1, wherein the first peripherallayer includes an inorganic material.
 5. The microsphere of claim 4,wherein the first peripheral layer includes silicon or titanium.
 6. Themicrosphere of claim 1, wherein the microsphere has a diameter of lessthan 500 micrometers.
 7. The microsphere of claim 1, wherein themicrosphere has a diameter of less than 10 micrometers.
 8. Themicrosphere of claim 1, wherein the microsphere has a diameter of lessthan 1 micrometer.
 9. The microsphere of claim 1, wherein the centralregion is substantially free of nanoparticles.
 10. The microsphere ofclaim 1, wherein the first nanoparticle is a metal nanoparticle, a metaloxide nanocrystal, or a semiconductor nanocrystal.
 11. The microsphereof claim 1, wherein the first nanoparticle is a semiconductornanocrystal.
 12. The microsphere of claim 11, wherein the semiconductornanocrystal includes a core including a first semiconductor material.13. The microsphere of claim 12, wherein the semiconductor nanocrystalfurther includes a shell overcoating the core, the shell including asecond semiconductor material.
 14. The microsphere of claim 13, whereinthe first semiconductor material is a Group II-VI compound, a Group II-Vcompound, a Group III-VI compound, a Group III-V compound, a Group IV-VIcompound, a Group I-III-VI compound, a Group II-IV-VI compound, or aGroup II-IV-V compound.
 15. The microsphere of claim 14, wherein thefirst semiconductor material is ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, HgS,HgSe, HgTe, AlN, AlP, AlAs, AlSb, GaN, GaP, GaAs, GaSh, GaSe, InN, InP,InAs, InSb, TlN, TlP, TlAs, TlSb, PbS, PbSe, PbTe, or mixtures thereof.16. The microsphere of claim 15, wherein the second semiconductormaterial is ZnO, ZnS, ZnSe, ZnTe, CdO, CdS, CdSe, CdTe, MgO, MgS, MgSe,MgTe, HgO, HgS, HgSe, HgTe, AlN, AlP, AlAs, AlSb, GaN, GaP, GaAs, GaSb,InN, InP, InAs, InSb, TlN, TlP, TlAs, TlSb, TlSb, PbS, PbSe, PbTe, ormixtures thereof.
 17. The microsphere of claim 11, wherein themicrosphere includes a plurality of semiconductor nanocrystals having arms deviation in diameter of no greater than 5%.
 18. The microsphere ofclaim 1, wherein the central region is substantially spherical in shape.19. The microsphere of claim 1, wherein the microsphere is a member of apopulation of microsphere having a rms deviation in diameter of nogreater than 10%.